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The Doctor
11-15-2012, 06:23 PM
I need to make essentially a very large washer (~3" OX x ~1.25 ID) out of clear black acrylic sheet (Perspex 962 Black).

I'll be using a CNC mill, with a spindle speed range of 80-30,000 RPM and feed to 147 IPM. Cutter will be a new, high quality 1/8" 4FL carbide end mill.

I would appreciate some advice on speeds, feeds, and techniques to do this without melting or excessive chipping. GWizard suggests ~20,000 RPM & ~147 IPM (.0019" per tooth). Feed per tooth stays the same at lower speeds. If I choose "conservative" on the slider, it drops to .0012 per tooth, but still 18,000 RPM+.

The recommended speeds seem likely to cause melting (no coolant) & the feed seems likely to chip. I'll probably use 3 passes to get through the full thickness, but not really sure about the rest. It's not a part where perfection is required, but I'm kinda picky.


Thanks
Ed

Evan
11-15-2012, 06:29 PM
Keep the rpm at the very low end like around 5000 rpm. Feed consistent with less than maximum chip load. If it is at all flexible make sure it cannot climb the cutter or it will. Lube with soapy water for cooling. BTW, I am having difficulty imagining "clear black" plastic.

John Stevenson
11-15-2012, 06:37 PM
Whoa, those speeds are way too high, Even is closer but to be honest I'd veer to a bit lower than that.
Can you use 2 flute or even a single flute router bit ?

Secret to milling plastic is to get rid of the chips, recutting chips causes heat and rewelding.

Just done three small machine windows today in 3mm clear perspex, ran at 20mm / sec but I cheated and used a laser, came out perfect with polished edges - sorry :)

Evan
11-15-2012, 06:52 PM
but I cheated and used a laser

You SUCK John. http://ixian.ca/pics9/biggrin.gif

If cutting dry I would also go lower than 5k rpm. With water it will work fine.

Georgineer
11-15-2012, 07:55 PM
Ed,

I'm currently working on a job which involves cutting some pretty complicated shapes in black acrylic. (Evan, Ed's not entirely wrong about Perspex being 'clear black' - when you mill it to less than 1 mm thickness and hold it up to the light, it's actually dark green and translucent!)

I did it on a much less powerful machine than yours and went for very conservative feed. I used a 2 mm 2-flute carbide endmill at 12 000 RPM without coolant, with 0.5 mm depth of cut and 8 mm/sec feed. The edges come out very smooth, 'semi-polished', without any hint of chipping. I agree with Evan about the importance of robust workholding.

My next challenge is to fake up a line bender with some nichrome wire and a power supply, to put an accurate 90 degree fold in it.

George

Evan
11-15-2012, 09:52 PM
Evan, Ed's not entirely wrong about Perspex being 'clear black' - when you mill it to less than 1 mm thickness and hold it up to the light, it's actually dark green and translucent!

Yeah, that holds for the dyed acrylic but not the opaque types. Both look the same with any reasonable thickness. I have many sq feet of acrylic and polycarb in a wide variety of colours that I picked up for nearly nothing when a local sign shop went out of business.

Rich Carlstedt
11-15-2012, 10:42 PM
Be careful with Acrylic !
You want Lexan or Plexiglass, which are Polycarbonates, and will not shatter suddenly, as Acrylic's will.
The difference is amazing

Rich

ckelloug
11-16-2012, 12:09 AM
Rich,

Plexiglass is just another acrylic. Makrolon (Bayer Materials Science) and Lexan (SABIC, formerly GE) are the two common polycarbonate brands.

--Cameron

browne92
11-16-2012, 12:16 AM
Not quite, Rich. Plexiglass is acrylic, or more specifically polymethyl methacrylate. Lexan is polycarbonate. Yes, plexi will shatter before Lexan, but I'm not sure where that fits in here. I've cut a lot of acrylic and never had it shatter. Shattering would be more a worry of the end product, and the good Doctor never said what that was.

I've been cutting and etching a lot of acrylic lately. I use a (is it 'zero' or 'oh'...looks like a flute to me, but that's what they call it.) flute router bit in my router table to cut it. The cuts come out clean, no chipping, but the finish isn't quite mirror like. Osrund makes the bits. I've tried standard router bits, but they just make a big melted mess.

I use a 2 flute end mill to etch it, but not without troubles.

Best of luck.

Edit: Cameron types faster than I do. :p

dalesvp
11-16-2012, 10:17 AM
Be sure to sandwich the acrylic between two stiff/rigid pieces of sacrifice materials such as good solid wood, squeeze TIGHT. Bore through the lot. This method prevents shattering of the fragile material while producing a perfect hole. I have successfully drilled paper, 1/8" acrylic, extremely fragile plastic bottle lids, tin foil and even cellophane food wrap.

Peter S
11-16-2012, 10:21 AM
I was just looking at a stack of laser cut acrylic tonight - a beautiful job, long thin strips, dead straight, perfect edges, much better than saw or other machined finish.

Rich Carlstedt
11-16-2012, 01:46 PM
Rich,

Plexiglass is just another acrylic. Makrolon (Bayer Materials Science) and Lexan (SABIC, formerly GE) are the two common polycarbonate brands.

--Cameron

Thanks, I stand corrected.
I always use Lexan for exotic parts.

Rich

Fasttrack
11-16-2012, 03:17 PM
Keep the rpm at the very low end like around 5000 rpm. Feed consistent with less than maximum chip load. If it is at all flexible make sure it cannot climb the cutter or it will. Lube with soapy water for cooling. BTW, I am having difficulty imagining "clear black" plastic.

LOL funny perspective. I'm so used to working with my old manual machines that 5000 sounds like the extreme high end! My fastest machine tops out at 5500 and my lathes top out at 1500 ... I would never think to call 5k the "very low end" :)

Bob Fisher
11-16-2012, 05:05 PM
Why would you not make this on a lathe? You can stack them and make a bunch at one time. For a washer, it would have been my first thought. Bob.

outlawspeeder
11-16-2012, 05:55 PM
If it is any help I cut Lexan with a wood router that has a bearing for following a pattern. The key to the cut is clearing the chips! If not they heat up...melt... clump... anf then all bad things happen.

I do this for motorcycle windshields. to finish the edge I use a file then buff to a nice class look.

Just a thought

Evan
11-16-2012, 07:18 PM
I have one spindle that does around 40k. I just bought an electronic tach that goes to 100k so now I can measure them without hauling out the frequency counter and a bunch of wires and a sensor etc. That will be very handy.

There is a very big difference between any acrylic including Plexy and polycarbonate (Lexan, a brand of PC). The materials may look similar but have very different properties in all areas.

BTW, Lexan was invented by GE and it was entirely accidental. A chemist mixed some polymer components while studying unrelated matters and not much happened.... Until the next day when the mess had set into and utterly indestructible mass.

A good substitute for polycarbonate is PET-G, especially in thin sections. It has many of the same properties, is much easier to heat form and is much cheaper. It also doesn't turn white when bent in a brake and it doesn't need to be heat dried before heat forming. No bubbles.

macona
11-17-2012, 02:37 AM
The single flute cutters are the best for plastic, especially the ones from Micro100. Another option is take a two flute end mill and grind off one flute. This gives you the extra clearance. Mist coolant works great here, the air keeps the chips from packing and the coolant keeps things from melting.

http://www.micro100.com/inch/sfp.html

John Stevenson
11-17-2012, 06:11 AM
The laser doesn't like Lexan, makes a horrible black blob so still have to mill this.
ABS cuts OK but need plenty of ventilation and when you look at the edge in cross section it not square but very slightly dumbell shaped.

Does toast brilliantly, you can even draw the shape where the eggs and bacon need to go..........................................

Evan
11-17-2012, 03:03 PM
You can do Lexan John. It needs to be dried first. Here is a very useful document from GE UK.

http://www.theplasticshop.co.uk/plastic_technical_data_sheets/lexan_polycarbonate_sheet_processing_guide.pdf

John Stevenson
11-17-2012, 03:42 PM
24 hours to dry 6mm, don't have the equipment to do this or need to do any amounts. Worth knowing though.

There are quite a few materials that can be engraved but not cut, the hard engraving laminate what we call Abec or Trafolite engraves well but burns when cutting.

Same with Tufnol or Phelonic, engraves well but burns.

Good thing is the plastics manufacturers are getting on the band waggon with the increase in low power cheap laser cutters and bringing new materials out all the while.

Trying a really cheap method this weekend on that Harrison M300 plate I posted about in this thread, method 4 actually.

http://bbs.homeshopmachinist.net/threads/56306